The borehole televiewer is a tool which records a circumferential "picture" of the borehole wall through the use of a rotating acoustic transducer. A sonic pulse fired from the transducer travels through the drilling mud until it impinges upon the borehole wall. A portion of the energy from the sonic pulse reflects off the wall and returns back to the transducer via the same path. This energy produces a voltage that is amplified, sent up-hole, and is recorded. A circumferential plot is produced at the surface by amplitude modulating the recorded voltages.
The purpose of a mud excluder is to shorten the distance the sonic pulse must travel through the drilling mud, since the mud is a dispersing medium which scatters the energy from the sonic pulse and ultimately contributes to decreased resolution of the televiewer. A conventional mud excluder is a rigid device machined from two pieces of brass. A rubber boot is fitted between the two parts to form a "window" for the pulse to pass through. Brine forms the transmission medium for the sonic pulse. Shear pins are fitted to the mud excluder to release it under the application of about 1000 lbs. static force in the event the excluder encounters an obstruction.
Since conventional mud excluders are rigid, and since any constrictions in the hole will shear them off, their outer diameters must be kept small with respect to the borehole diameter. This small outer diameter means that the travel path of the sonic pulse through the mud is longer than it would be if the outer diameter was larger. The net result of the small outer diameter is decreased resolution.
Since conventional mud excluders are not pressure-compensated, some deformation occurs in the rubber boot window. This deformation reduces the effectiveness of the mud excluder in two ways. First, the deformation increases the travel path of the sonic pulse through the mud. Second, since the window tends not to deform uniaxially, the pulse may impinge upon the window non-perpendicularly, causing some reflection of the signal.